At one observation point in Dinghu Mountain, Zhaoqing , more than a dozen plant species growing below an old tree had died off until only one or two were left, and the tree could be next to go if the "nitrogen fallout" from smog continued, Lu said.

"Immediate measures must be taken to reduce air pollution, especially nitrogen emissions," Lu said.

"If the situation remains as it is, most forests in southern China will be destroyed within decades. But the impact is not limited in China. The problem will have a ripple effect around the world."

The study, published in this month's journal, run by the American Chemical Society, said the scientists took more than a decade to find solid evidence that smog is killing off trees.

Nitrogen is one of the most important causes for the formation of smog. Many human activities, such as industrial production and vehicle exhaust emissions, pump large quantities of nitrogen into the atmosphere.

Nitrogen is a form of nutrition for plants, so farmers have used nitrogen-rich fertilisers to boost agricultural production.

Common sense might suggest airborne nitrogen would benefit all kinds of plants after being absorbed by the roots after falling to the soil in train or dust.

Yet the Chinese study suggests otherwise.

Lu said old natural forests were originally so full of nitrogen that they could not absorb more.

"It is like a bowl full of water: adding more water will only cause some to spill out," he said.

It was this "nitrogen spill" that was killing forest areas, he added. The nitrogen found in smog usually settled on the ground in an acid form, such as nitric acid, which would bind with other elements such as calcium, aluminium, magnesium and then flow away in water, he said.

This process, invisible to the naked eye, was detected only after observations over a long period and extensive laboratory tests. The scientists discovered it had led to the rapid loss of biodiversity in well-protected primary forest areas in Guangdong.

"I'm afraid the forests in Hong Kong are experiencing the same problem," Lu said. "They are in similar environment and affected by more or less the same level of air pollution."

China's problem of "nitrogen fallout" is among the worst in the world. In Guangzhou, more than 70kg of nitrogen falls an every 0.4 hectare of land each year as a result of air pollution, and the situation is similar in other areas.

The high nitrogen levels now "denuding" the soil in Chinese natural forest areas have provided a new explanation for the widespread disappearance of mainland forests in recent years.

At least 40 per cent of the natural forest areas of Hainan province, for instance, have disappeared between 2001-11, sparking debate on the cause.

Environmentalists blamed economic activities such as the work of rubber tree plantations, but local authorities have always insisted that natural forests were protected from external disturbances by law.

Lu said one solution would be challenging but effective. China and nations needed to agree to cut nitrogen emissions.

Other nations had to be aware that the problem affected them too, he said. Nitrogen fallout could easily be carried by winds over vast distances, for example, from China as far as the western coast of the United States.